The formation of superoxide radicals (02.) at the mitochondrial respiratory chain is the quantitatively most important source of oxidative stress (Beckman and Ames, Physiol. Rev. 78 (1998) 547-581). The superoxide radical is formed by univalent reduction of molecular oxygen at the ubisemiquinone component of the respiratory chain (FIG. 2). The oxidative stress is relatively low if electrons can flow in an uninhibited way through the components of the respiratory chain without forming superoxide radicals. In the course of this process, the electrons pump protons against the membrane potential of the mitochondrial matrix from the mitochondrial matrix into the extramitochondrial space and convert at the end of the respiratory chain molecular oxygen into water with the help of cytochrome C oxidase. The chemical energy of the proton gradient is then used by the ATP synthetase complex of the mitochondrial matrix to convert adenosine diphosphate (ADP) into adenosine triphosphate (ATP) (see FIG. 2). If the demand of chemical energy of the cell is low and all ADP is converted into ATP, the flux of protons back into the mitochondrial matrix is also accordingly low. In such a condition; the protons remain in the extramitochondrial space and inhibit thereby the flux of the electrons to the respiratory chain. Accordingly, the mitochondrial membrane potential increases to the point where the normal flux of electrons all the way through the respiratory chain to the cytochrome C oxidase becomes energetically less favorable and the alternative transfer of electrons to molecular oxygen at the ubisemiquinone component (which yields superoxide radicals) becomes energetically more favorable. The undesirable production of radicals and the corresponding waste of energy is limited in intact cells at least to some extent by a negative feedback mechanism (FIG. 2). ATP concentrations inhibit the enzyme phosphofructo-kinase (PFK) of the glycolytic metabolism and block thereby the availability of mitochondrial energy substrates such as NADH (FIG. 2). This mechanism downregulates the influx of electrons into the mitochondrial respiratory chain and suppresses the further increase in the mitochondrial proton gradient which would otherwise facilitate the transfer of electrons to molecular oxygen and the corresponding formation of superoxide radicals. Certain cell types such as skeletal muscle cells and cells of the nervous tissue have, in addition, the ability to accumulate relatively high intracellular concentrations of creatine which in conjunction with the enzyme creatine kinase converts ATP back into ADP at the mitochondrial matrix. The resulting phosphocreatine is transported more efficiently than ATP to other points in the cytoplasm, including the site of the glycolytic metabolism (FIG. 2).
The manifestation of oxidative stress in the blood plasma can be described by two easily detectable components namely the decrease in the plasma concentration of acid-soluble thiol (essentially reduced cysteine) and by the increase of the plasma concentration of cysteine disulfide (cystine) (FIG. 1). The ratio of these two components is an indicator of the thiol/disulfide redox state and is a manifestation of oxidative stress that can be easily tested (Gohil et al., L Appl. Physiol. 64 (1988) 115-119; Duthie et al., Arch. Biochem. Biophys. 282 (1990) 78-83; Sastre et al., Am. J. Physiol. 263 (1992) R992-R995; Sen et al., J. Appl. Physiol. 76 (1994) 2570-2577; Hack et al., BLOOD 92 (1998) 59-67). In the course of senescence (FIG. 1) and certain disease conditions including cancer, the loss of body cell mass and skeletal muscle function is associated with an increased oxidative stress and a significant prooxidative shift in the plasma thiol/disulfide redox state (Hack et al., BLOOD 92 (1998) 59-67). Because many physiological processes are regulated by redox-sensitive signalling cascades, which respond either to changes in the concentration of reactive oxygen species or to changes in the thiol/disulfide redox state, the shift in the plasma redox state may play a causal role in the aging process and its related degenerative consequences. These aging-related degenerative processes include especially the age-related loss of skeletal muscle mass and muscle function.
Reactive oxygen species which are produced in substantial amounts by various NADPH oxidase isoforms in many different cell types of the body play an important role in various signalling processes. Many of these redox-sensitive signalling cascades respond also to changes in the thiol/disulfide redox state. Amongst other examples it has been shown that the “replicative senescence” of cells can be induced either by reactive oxygen species or by a prooxidative shift in the thiol/disulfide redox state.
Numerous studies suggest that the aging process and various disease-related degenerative processes are caused, at least partly, by the free-radical-mediated oxidative stress and/or the oxidative shift in the thiol/disulfide redox state (Beckaman and Ames, Physiol. Rev. 78 (1998) 547-581; Dröge, Physiol. Rev. 2002, in press). Oxidative stress has also been implicated in the development of neurodegenerative diseases, especially Alzheimer's disease (Montine et al., J. Neuropathol. Exp. Neurol. 56 (1997) 866-871; Sayre et al., J. Neurochem 68 (1997) 2092-2097; Lovell et al., Neurobiol. Aging 18 (1997) 457-461; Multhaup et al., Biochem. Pharmacol. 54 (1997) 533-539; Praticö et al., FASEB J. 12 (1998) 1777-1783; Behl et al., Cell 77 (1994) 817-827; Kaltschmidt et al., Proc. Natl. Acad. Sci. USA 94 (1997) 2642-2647), and amyotrophic lateral sklerosis (Rosen et al., Nature 362 (1993) 59-62; Tu et al., Lab. Invest. 76 (1997) 441-456). Moreover, studies on primates revealed a massive age-related increase in oxidative stress in the skeletal muscle tissue (Zainal et al.; FASEB J. 14 (2000) 1825-1836), arid clear manifestations of oxidative stress were also seen in gene expression profiles of skeletal muscle tissue and brain tissue from old mice as detected by oligonucleotide arrays (Lee et al., Science 285 (1999) 1390-1393; Lee et al., Nature Genet. 25 (2000) 294-297. Experimental animal studies have finally shown that the thiol/disulfide redox state of the blood is correlated with the intracellular glutathione redox state (Ushmorov et al., Cancer Res. 59 (1999) 3527-3534).